A transmission or gearbox is a system which transmits mechanical power from a prime mover, e.g. an engine or electric motor, to some form of output device. Typically, the rotational speed of an input shaft is changed, resulting in a different output speed. However, some transmissions merely change the physical direction in which power is transmitted.
Planet gear systems comprise one or more planet gears rotating about a centrally located sun gear. Sometimes, the planet gears are mounted on a movable carrier. The carrier may either be fixed relative to the housing, or it may rotate relative to the housing and/or relative to the sun gear. The transmission may further incorporate an cuter ring gear with radially inwardly projecting gear teeth, generally referred to as the annulus.
In traditional planet gears, the ability to shift between different gear ratios may require a complicated wheel configuration.